This invention relates to optoelectronics devices and fabrication method particularly to light emitting diodes and laser diodes.
Light emitting diodes are widely used in optical displays, traffic lights, data storage, communications, medical and many other applications.
The development of blue LEDs and laser diodes has attracted considerable research activity to the growth of group III-nitrides. The band gap of group III-nitrides can be varied to provide light over nearly the whole spectral range from near UV to red. Accordingly, group III-nitrides find use in active regions of these devices.
The growth of InxGa1xe2x88x92xN alloys and quantum wells is extremely difficult mostly due to the trade-off between the epilayer quality and the amount of InN incorporation into the alloy. Growth at high temperatures of approximately 800xc2x0 C. typically results in high crystalline quality but the amount of InN in the solid is limited to low values because of the high volatility of indium. Lowering the growth temperature results in an increase in the indium content at the expense of reduced crystalline quality. The lattice mismatch and different thermal stability of the two constituents, InN and GaN, also complicate the growth of InxGa1xe2x88x92xN. The lattice mismatch can lead to a miscibility gap [Ho and G. B. Stringfellow, Appl. Phys. Lett. 69, 2701 (1996).], which causes fluctuations of In content across the film. Singh and co-workers[R. Singh, D. Doppalapudi, T. D. Moustakas, and L. T. Romano, Appl. Phys. Lett. 70, 1089 (1997); R. Singh and T. D. Moustakas, Mater. Res. Soc. Symp. Proc. 395, 163 (1996); R. Singh, W. D. Herzog, D. Doppalapudi, M. S. Unlu, B. B. Goldberg, and T. D. Moustakas, Mater. Res. Soc. Symp. Proc. 449, 185 (1997).] provided strong evidence of phase separation in InGaN thick films grown by molecular beam epitaxy (MBE). Other researchers reported phase separation in thick InGaN films grown by metalorganic chemical vapor deposition (MOCVD) [Akihiro Wakahara, Takashi Tokuda, Xiao-Zhong Dang, Susumu Noda, and Akio Sasaki , Appl. Phys. Lett. 71, 906, (1997); N. A. El-Masry, E. L. Piner, S. X. Liu, and S. M. Bedair Appl. Phys. Lett., 72, 40, (1998).]. Behbehani [M. K. Behbehani, E. L. Piner, S. X. Liu, N. A. El-Masry, and S. M. Bedair Appl. Phys. Lett. 75 2202, (1999)] reported the coexistence of phase-separation and ordering in InxGa1xe2x88x92xN with x greater than 0.25. Up to now, growth of InGaN/GaN quantum wells (QW) with emission in the green is still a challenging task.
InGaN is a very important material because it is used in the active layer of LEDs and laser diodes (LD), However, researchers have not reached consensus: on the optical emission mechanism in InxGa1xe2x88x92xN/InyGa1xe2x88x92yN QWs. There are a few theories; one attributing emission to In-rich quantum dots (QDs), one attributing emission to the piezoelectric effect and another combining aspects of both. Indium-rich QDs can be formed by spinodal decomposition, Stranski-Krastanov (SK) growth mode, or using antisurfactants.
It is therefore an object of the present invention to provide an improved technique for the growth of self-organized InGaN quantum dots.
It is another object of the invention to produce light emitting diodes capable of emitting blue and green light.
These objects are provided, according to the present invention, by indium-rich QDs which are embedded in InxGa1xe2x88x92xN/GaN or InxGa1xe2x88x92xN/InyGa1xe2x88x92yN single and multiple quantum wells. These QDs are triggered by the flow of trimethyl indium (TMIn) or other indium precursors acting as antisurfactants on a non-growing surface.
The conventional method of growth of QW is the following:
First, growing a low temperature buffer and then a high temperature GaN layer, with the former usually performed in the range of 450xc2x0 C. to 600xc2x0 C. and the latter usually performed in the range of 900xc2x0 C. to 1100xc2x0 C., most typically at 1030xc2x0 C. The temperature was next lowered to about 700xc2x0 C. to 800xc2x0 C. to grow the GaN or InGaN barrier followed by the growth of the quantum well.
In this invention, after the growth of the barrier, an appropriate amount of indium-precursor such as trimethyl indium (TMIn), or triethyl indium (TEIn) or ethyldimethyl indium (EDMIn), was flowed in the presence of ammonia. Indium atoms from indium-precursor aggregate at the atomic edges of the InGaN barriers to form the xe2x80x9cseedsxe2x80x9d for the subsequent growth of quantum dots. So long as these precursors are used, whether be it in MOCVD or chemical beam epitaxy (CBE), this invention is applicable.
In a preferred embodiment of the present invention, a group of INxGa1xe2x88x92xN/InyGa1xe2x88x92yN single quantum wells (SQWs) were grown by MOCVD on (0001) sapphire substrates. MOCVD was performed using trimethyl gallium (TMGa), trimethyl indium (TMIn), and ammonia (NH3) as precursors, and hydrogen (H2) and/or nitrogen (N2) as the carrier gases. Triethyl gallium (TEGa), ethyldimethyl gallium (EDMGa), triethyl indium (TEIn), ethyldimethyl indium (EDMIn) can also be used as group III precursors, while dimethylhydrazine (H2N2(CH3)2. 1,1 DMHy) is preferred as a N precursor. For this group of SQWs, a 2 xcexcm thick undoped bulk GaN was first grown on a 250 xc3x85 thick GaN buffer layer. The growth temperature was 530xc2x0 C. and 1030xc2x0 C. respectively for the GaN buffer and bulk layer. After deposition of the GaN bulk layer, the growth temperature was lowered down to about 700xc2x0 C. to 800xc2x0 C. for the deposition of the InxGa1xe2x88x92xN barrier and the InyGa1xe2x88x92xN well. After the deposition of the InxGa1xe2x88x92xN barrier wherein x ranges from 0 to 0.10, and prior to the growth of InyGa1xe2x88x92yN well, wherein y is greater than x, TMIn was flowed at a flow rate less than 100 xcexcmol/min for a short time varying from 2 to 5 seconds with TMGa flow switched off.
The well thickness was about 30 xc3x85. A high temperature cap layer was grown on the top of InxGa1xe2x88x92xN/InyGa1xe2x88x92yN SQW at temperatures in the range of from 800xc2x0 C. to 1030xc2x0 C.
In the second embodiment of the present invention, the second group of SQWs was grown, the growth conditions are the same as the first except that before the growth of the InGaN barrier, a low temperature GaN layer was grown at the same temperature as the growth temperature of the barrier and well so that no temperature ramping is needed for the subsequent growth.
According to the invention, it has been found that the photoluminescence from the first and second groups of SQWs are 488 nm and 548 nm respectively at the room temperature. The luminescence from the first and the second group are in the blue and green regions respectively, which are suitable for the fabrication of blue and green LEDs.
The amount of TMIn acting as antisurfacants and the duration of the TMIn flow are important for the growth of indium-rich QDs: too small a flow may not form enough xe2x80x9cseedsxe2x80x9d for the subsequent growth of the QDs, while too much flow will create indium droplets which are competing with the formation of indium-rich QDs. At room temperature, the luminescence comes from these dots rather than from the wells. The quantum confinement effect of the QDs is the reason why QDs have very high efficiency of luminescence even at room temperature.
After the flowing of TMIn, which acts as antisurfactant, the subsequent flow of TMIn, TMGa and ammonia are also very important for the growth of QDs.